Journal of Experimental Botany, Vol. 49, No. 324, pp. 1183–1190, July 1998
Role of uranium speciation in the uptake and
translocation of uranium by plants
Stephen D. Ebbs, Danielle J Brady and Leon V. Kochian1
US Plant, Soil, and Nutrition Laboratory, USDA-ARS, Cornell University, Ithaca, NY 14853, USA
Received 10 November 1997; Accepted 16 February 1998
Abstract
Uranium (U) uptake and translocation by plants was
characterized using a computer speciation model to
develop a nutrient culture system that provided U as
a single predominant species in solution. A hydroponic
uptake study determined that at pH 5.0, the uranyl
(UO2+) cation was more readily taken up and transloc2
ated by peas (Pisum sativum) than the hydroxyl and
carbonate U complexes present in the solution at
pH 6.0 and 8.0, respectively. A subsequent experiment
tested the extent to which various monocot and dicot
species take up and translocate the uranyl cation. Of
the species screened, tepary bean (Phaseolus acutifolius) and red beet (Beta vulgaris) were the species
showing the greatest accumulation of U. In addition to
providing fundamental information regarding U uptake
by plants, the results obtained also have implications
for the phytoremediation of U-contaminated soils. The
initial characterization of U uptake by peas suggested
that in the field, a soil pH of <5.5 would be required
in order to provide U in the most plant-available form.
A pot study using U-contaminated soil was therefore
conducted to assess the extent to which two soil
amendments, HEDTA and citric acid, were capable of
acidifying the soil, increasing U solubility, and enhancing U uptake by red beet. Of these two amendments,
only citric acid proved effective, decreasing the soil
pH to 5.0 and increasing U accumulation by a factor
of 14. The results of this pot study provide a basis for
the development of an effective phytoremediation
strategy for U-contaminated soils.
Key words: Uranium, citrate, phytoremediation, uptake,
speciation.
Introduction
The US Department of Energy ( USDOE ) has for decades
been the organization most heavily involved with the
nation’s uranium ( U ) resources. Unfortunately, emissions
and accidental spills have led to soil U contamination at
a number of USDOE research laboratories and commercial mining facilities. More than 50% of USDOE facilities
involved in reactor operations, weapons research, nuclear
fuel production, and waste reprocessing reported that U
was the most frequent radionuclide contaminant in
groundwater and surface soils (Riley et al., 1992).
U-contamination at the former Feed Materials
Production Center at Fernald, OH., led to the addition
of this area to the US Environmental Protection Agency’s
National Priorities List as a ‘Superfund’ site. Remediation
of these contaminated soils is a high priority within the
USDOE primarily because these soils represent a potential
source of groundwater contamination (Lee et al., 1993).
As is the case for heavy metals, the current remediation
technology for U-contaminated sites most often involves
identification of the contaminated soils, mass excavation
of these areas, and packaging and removal of the soil to
disposal facilities. However, this technique is expensive
and requires long-term monitoring. As a result, USDOE
has initiated restoration projects such as the ‘Uranium in
Soils Integrated Demonstration’ ( USID) to evaluate and
compare the versatility, efficiency, and economics of various technologies for the removal of U from contaminated
soils (Lee and Marsh, 1992). Phytoremediation, the use
of terrestrial plants to remove, or phytoextract, contaminants from polluted soil, is one technology being evaluated.
The development of a phytoremediation strategy for
any contaminated site requires, among other things, an
understanding of the geochemical behaviour of the contaminant in the soil, knowledge of the chemical species
of the contaminant most readily accumulated by plants,
the identification of a plant species capable of both taking
up and translocating that chemical species, and the
development of agronomic and/or amendment strategies
designed to maximize the availability and uptake of the
chemical species of interest. For heavy metals such as Cd,
1 To whom correspondence should be addressed. Fax: +1 607 255 2454. E-mail: [email protected]
© Oxford University Press 1998
1184
Ebbs et al.
Ni, and Zn, current research in the field of phytoremediation has provided much of this information.
Unfortunately, such fundamental information is lacking
for U.
The pH-dependent speciation of U in soil and aqueous
systems is an area that has been extensively studied. U is
present in the soil primarily (80–90%) in the +VI oxidation state as the uranyl ( UO2+) cation (Bondietti and
2
Sweeton, 1977; Sheppard, 1980; Sheppard and Evenden,
1988; Allen et al., 1994; Mortvedt, 1994), despite the fact
that the contamination at sites like the Fernald, OH.,
USDOE facility originated as tetravalent UO (Allen
2
et al., 1994). Under acidic, reducing conditions, UO2+ is
2
the predominant U species in the soil ( Hostetler and
Garrels, 1962; Langmuir, 1978; Mortvedt, 1994). Hydroxide complexes, such as UO OH+, ( UO ) (OH )2+,
2
2 2
2
( UO ) (OH )+, and ( UO ) (OH )− and phosphate com2 3
5
2 3
7
plexes such as UO HPO0 and UO ( HPO )2−, form under
2
4
2
4 2
near neutral conditions (Langmuir, 1978), while carbonate complexes such as UO CO0, UO (CO )2−, and
2 3
2
3 2
UO (CO )4− predominate under alkaline conditions
2
3 3
(Langmuir, 1978; Lee et al., 1993; Mortvedt, 1994). The
stability of these complexes vary (Mortvedt, 1994), but
UO (HPO )2− is the most stable complex from pH 4–7.5
2
4 2
(Langmuir, 1978; Lee et al., 1993).
There is little information relating U speciation to plant
uptake. The form(s) of U taken up by plants and the
mechanism by which this occurs have yet to be identified.
Most of the work to date has focused on either the U
content of native plants growing on U-contaminated soils
( Ibrahim and Whicker, 1988; Saric et al., 1995) or U
accumulation by field and garden crops of importance to
animals and humans (Sheppard et al., 1984; Lakshmanan
and Venkateswarlu, 1988; Sheppard et al., 1989). These
studies do not, however, provide the information
necessary to develop a phytoremediation strategy for
U-contaminated soils.
The present study was undertaken to provide fundamental information regarding the uptake and translocation of U by plants and to utilize that information to
improve the phytoextraction of U from a contaminated
soil. The first part of this study used computer speciation
modelling and hydroponic experiments to determine the
form of U most readily accumulated in plant shoots. This
was accomplished by buffering nutrient solutions at various pH values so as to expose plants to a single predominant U species, rather than a mixture of U species. This
allowed for an assessment of both the uptake and the
toxicity of U species. The results indicated that the free
uranyl ( UO2+) cation, which predominates at a pH of
2
5.0–5.5, was the form of U most readily accumulated by
plants. With this established, numerous plant species were
then screened hydroponically for the ability to both take
up and translocate the uranyl cation. A variety of species
were tested, including both monocot and dicot species.
Among those included were Brassica juncea and Brassica
rapa, two species which has recently been shown to
accumulate heavy metals such as Cd, Cu, Ni, Pb, and Zn
( Kumar et al., 1995; Ebbs et al., 1997). Corn and oats
were included as representative high-biomass monocot
species. Finally, using one of the plant species identified
by the screening experiments and a U-contaminated soil
obtained from a contaminated site near Ashtabulah, OH.,
a pot experiment was conducted to assess the extent to
which red beet was capable of phytoextracting U from
contaminated soil. This research involved the use of weak
organic acids that could both lower soil pH to convert
most of the U to the uranyl cation, and increase the
bioavailability of U for plant uptake.
Materials and methods
Design of hydroponic culture system
The modified Johnson’s nutrient solution used in the hydroponic
experiments described below consisted of the following: 3.0 mM
KNO ; 2.0 mM Ca(NO ) ; 0.5 mM MgSO ; 25 mM KCl;
3
3 2
4
12.5 mM H BO ; 1.0 mM MnSO ; 1.0 mM ZnSO ; 0.5 mM
3 3
4
4
CuSO ; 0.1 mM H MoO ; and 0.1 mM NiSO . Iron (5 mM )
4
2
4
4
was supplied to dicots as Fe-EDDHA (N,N∞-ethylenebis[2-(2-hydroxyphenyl-glycine]) and to grass species as
Fe-HEDTA (N-hydroxylethylethylenediaminetriacetic acid ).
Using this nutrient composition as a basis, GEOCHEM-PC
(Parker et al., 1995) was used to model nutrient solutions which
provided a good separation of U complexes. The normal P
source for this nutrient solution, 0.1 mM NH H PO , was
4 2 4
excluded from the model to preclude the formation of uranyl
phosphate complexes. The goal was to develop a hydroponic
system that provided U predominantly as either the free uranyl
cation, uranyl hydroxides, or uranyl carbonates.
Uptake of individual U species
Seeds of peas (Pisum sativum cv. ‘Sparkle’) were germinated on
moistened filter paper for 4 d and transferred to 2 l pots
containing the nutrient solution described above. Three pH
treatments were imposed by addition of 2 mM 2-[N-morhpolino]ethane-sulphonic acid (MES) (Sigma, St Louis, MO.), titrated
to pH 5.0 or 6.0, or the addition of 1 mM KHCO (pH 8.0).
3
There were four replicates of each treatment. Plants were precultured in a growth chamber with a 16 h photoperiod for 10 d
in the presence of 0.1 mM NH H PO . After this time, plants
4 2 4
were removed from the solution, the roots rinsed in deionized
water and the plants transferred to solutions containing nutrient
solution of the same composition except that P was excluded
and 5 mM UO (NO ) was included. The control treatment
2
3 2
consisted of nutrient solution lacking both P and U. Plants
were grown for an additional 7 d before roots and shoots were
harvested. Roots were rinsed five times in deionized water.
Root and shoot tissues were dried and digested with nitric acid
at 180 °C for more than 2 h followed by 151 nitric/perchloric
acid at 220 °C until the sample was completely digested. The
ash was resuspended in 5% nitric acid and analysed using an
inductively-coupled argon plasma Emission Spectrometer
(ICAP-ES) (Model 61E, Thermo Jarrell Ash, Franklin, MA).
Statistical analyses of the results from this and all subsequent
experiments utilized the Student’s t-test at the 0.05 level of
significance.
Uranium speciation and uptake
Effect of P on the uptake of UO2+
2
Pea seedlings were grown hydroponically as in the previous
experiment for 10 d in the presence of 100 mM P. After the
preculture period, plants were transferred to one of four
treatments: control (no-U, no-P); 5 mM U no-P; no-U+5 mM
P; and 5 mM U+5 mM P. Each treatment was replicated four
times. Plants were grown for an additional 7d, with roots and
shoots harvested and analysed as in the previous experiment.
Screening of plant species for U accumulation
The following species were screened for the ability to accumulate
U: peas (Pisum sativum L. cvs ‘Sparkle’ and ‘E107’); two
varieties of Indian mustard (Brassica juncea L.), accessions
426308 ( USDA-ARS Plant Introduction Center, Iowa State
University) and cv. ‘RH–30’ (Mycogen Plant Sciences, Madison,
WI ); turnip (B. rapa L., accession 163496) ( USDA-ARS Plant
Introduction Center, Iowa State University); red beet (Beta
vulgaris L. cv. ‘Sweetheart’, Johnny’s Seeds, Albion, ME);
alfalfa (Medicago sativa L., cv. Germain WL512); common
vetch (Vicia sativa L.) and hairy vetch (Vicia villosa L.) (cvs
unknown, F. and J. Seed Service, Woodstock, IL.), crown vetch
(Coronilla varia L., cv. ‘Chemung’, Granite Seed, Lehi, UT ),
tepary bean (Phaseolus acutifolius L., var. L567, U.C. Riverside
Legume Collection, Riverside, CA), oats (Avena sativa L., cv.
‘Otana’, Treasure St. Seed, Fairfield, MT ), and corn (Zea mays
L., var. 3377, Pioneer Seed, Johnston, IA). Nomenclature for
the above species follows Terrell et al. (1986). Seedlings were
grown as in the first experiment, a 10 d preculture period in the
presence of P followed by exposure to 5 mM U in the absence
of P, with four replicate plants for each species. Plants were
harvested and analysed in the same manner as in previous
experiments.
Uptake of U from contaminated soil
A pot study was conducted to assess the extent of U uptake by
red beet. U-contaminated soil was provided by RMI
Environmental Services (Ashtabula, OH ). The soil was sieved
through a 5 mm steel sieve and stored in plastic barrels at a
moisture level close to that at which the soil had been collected.
The soil pH of 6.8 was determined from a 251 (by vol.) 0.01 N
CaCl /air-dry soil slurry that had been shaken for 10 min. Total
2
U content of the soil (310 mg U kg−1) was determined by
digesting the soil using the procedure described above followed
by ICAP-ES analysis.
Beet seeds were germinated for 2–3 d on moistened filter
paper. Beets were grown in potting mix for an additional
7 d before being transplanted to pots containing 500 g of
U-contaminated soil. Seedlings were watered with the same
nutrient solution used in the hydroponic experiments, buffered
to the soil pH of 6.8 using 1 mM N-2-hydroxyethylpiperazineN∞-2-ethansulphonic acid (HEPES ). To prevent the development
of P deficiency, plant shoots were sprayed twice each week with
10 mM KH PO adjusted to pH 6.0 with KOH. During foliar
2 4
P application, the soil surface was covered to prevent the
introduction of P into the soil. Care was also taken to ensure
that the foliar P solution did not flow down the stem. The beets
were grown for an additional 5 weeks.
During the last week of growth, subsets of pots (5 replicates
per treatment) were treated with one of two soil amendments.
The treatments consisted of the addition of 50 ml of either
0.09 mM HEDTA (trisodium salt) or 0.25 M citric acid,
followed by a second application of the same solutions 3 d
later. These treatments raised the soil concentration of HEDTA
to 5 g kg−1 soil and the citric acid concentration to 10.5 mg kg−1
soil. This concentration of HEDTA is comparable to that used
1185
in studies artificially inducing hyperaccumulation from a heavy
metal-contaminated soil (Huang and Cunningham, 1996; Huang
et al., 1997). The citric acid concentration used in this study
was based upon results obtained from other experiments
investigating the kinetics of U solubilization using this
U-contaminated soil mixture ( Ebbs et al., unpublished results).
Deionized water was added to a third subset of pots as a
control. After this additional week of growth, plant shoots were
harvested and analysed as in the previous experiments.
Soil samples were taken from each pot at harvest. Soil pH
was measured to determine if the amendments lowered the pH
to the target value of 5.0–5.5. The soil samples were extracted
with water to determine the effect of the amendments on U
solubility.
Results
Hydroponic culture system development
GEOCHEM-PC modelling indicated that the best resolution of U species was achieved at pH values of 5.0, 6.0
and 8.0. At pH 5.0, nearly 80% of the U was present in
solution as the free uranyl ( UO2+) cation ( Fig. 1). At
2
pH 6.0, 94% of the U was present as hydroxide complexes,
while at pH 8.0, nearly 100% of the U was present as
carbonate complexes. As expected, the modelling also
indicated that in the presence of P, uranium phosphate
complexes would be stable over the pH range from 4.5
to 9.0 (Fig. 2). The formation of the U-phosphate complexes was predicted to reduce the level of free uranyl
cation and uranyl hydoxides by approximately 10%. At
pH >7.0 in the presence of P, the model indicated that
U-hydroxyl precipitates would form rather than the carbonate complexes that had been predicted in the absence
of P.
Fig. 1. U speciation in a modified Johnson’s nutrient solution in the
absence of P, as predicted by GEOCHEM-PC.
1186
Ebbs et al.
The uptake of U into shoots of peas was influenced by
pH, presumably because of the differences in the form of
U in solution. The greatest shoot U concentration and
accumulation occurred at pH 5.0 when U was present
predominantly as the free uranyl cation (Figs 4, 5).
Uptake at pH 6.0 was less than 20% of that at pH 5.0
while uptake at pH 8.0 was about 5% of that at pH 5.0.
These results suggest that the cationic uranyl ion is the
species most readily taken up and translocated by peas.
As a desorption procedure was not carried out, concentrations of U in roots may include U adsorbed to root cells
walls as well as what was taken up. Root U concentrations
Fig. 2. U speciation in a modified Johnson’s nutrient solution in the
presence of 5 mM P, as predicted by GEOCHEM-PC.
Uptake of individual U species
In the absence of U, shoot dry weights of pea were not
significantly different at the three pH values ( Fig. 3). The
root dry weight at pH 5.0 was less than that at pH 6.0
and 8.0. In the presence of U, shoot dry weight increased
with increasing pH, but root weight was significantly
higher at pH 5.0 than at 6.0 or 8.0. Shoot dry weights
were higher in the presence of U at all pH levels.
Observations of roots at harvest revealed that 5 mM U
was toxic to roots at all three pH levels. The symptoms
were similar to those for Al toxicity, including stunting
of lateral root growth and blackening of existing root
apices. The effect was most pronounced at pH 6.0.
Fig. 3. Shoot and root dry weights for pea (cv. ‘Sparkle’) plants exposed
to 5 mM U for 7 d at three different pH values. Error bars denote the
standard error of the mean (n=4).
Fig. 4. Uranium concentration in roots and shoots of peas (cv.
‘Sparkle’) exposed to 5 mM U for 7 d at three different pH values. Error
bars denote the standard error of the mean (n=4).
Fig. 5. Uranium accumulation in roots and shoots of peas (cv. ‘Sparkle’)
exposed to 5 mM U for 7 d at three different pH values. Error bars
denote the standard error of the mean (n=4).
Uranium speciation and uptake
1187
were generally higher than those for shoots. Uptake/
adsorption by roots was greatest at pH 6.0 and 8.0, with
a lower concentration at pH 5.0 (Figs 4, 5).
Effect of P on the uptake of UO2+
2
In the absence of U, there was no significant difference
in root or shoot dry weights for pea plants grown for 7d
in the presence or absence of P at pH 5.0 ( Fig. 6). Shoot
P concentrations for control plants also did not differ
between the two treatments (data not shown), suggesting
that the plant had sufficient P reserves after the preculture
period to allow for normal growth and development
during the exposure to U.
In the presence of 5 mM U, in the absence of P, growth
of both roots and shoots was severely inhibited compared
to the –U-grown control plants. In solutions containing
both U and P, root and shoot dry weights did not differ
significantly from the –U control plants, suggesting that
P had largely overcome the toxic effects of U in solution,
most likely due to complexation of the U with phosphate.
Complexation may have also reduced the bioavailability of U to peas, as there was a >50% reduction in
the U concentration of roots and shoots at pH 5.0
( Fig. 7). Roots of peas in solution containing both U
and P still displayed symptoms of toxicity, but the dry
weights were similar. This may have been due to the fact
that lateral roots for the plants grown in the presence of
both U and P were stunted and thickened, compensating
for the reduction in lateral root length.
Screening of plant species for U accumulation
On a concentration basis, all the species screened in this
experiment exhibited a greater accumulation of U than
Fig. 6. Shoot and root dry weights for pea (cv. ‘Sparkle’) plants exposed
to 5 mM U for 7 d at pH 5.0 in the presence and absence of 100 mM P.
Error bars denote the standard error of the mean (n=4).
Fig. 7. Uranium concentration in roots and shoots of peas (cv.
‘Sparkle’) exposed to 5 mM U for 7 d at pH 5.0 in the presence and
absence of 100 mM P. Error bars denote the standard error of the mean
(n=4).
peas. The highest concentrations were found in beet and
crown vetch ( Fig. 8). On a per plant basis total U
accumulation was greatest for beet and tepary bean
( Fig. 9). Species with small seeds showed symptoms of P
deficiency during the seventh day of treatment, suggesting
that the P reserves within the plant had been exhausted.
This deficiency, however, could be overcome through
foliar P fertilization. This method could be used to
provide P to developing plants without the need to include
P in the growth solution.
Fig. 8. Uranium concentration in shoots of several species exposed to
5 mM U for 7 d at pH 5.0. Error bars denote the standard error of the
mean (n=4).
1188
Ebbs et al.
Fig. 9. Uranium accumulation in shoots of several species exposed to
5 mM U for 7 d at pH 5.0. Error bars denote the standard error of the
mean (n=4).
Fig. 11. Shoot biomass for beet grown for 5 weeks in pots of either
unamended U-contaminated soil or soil amended with HEDTA or
citric acid. Error bars denote the standard error of the mean (n=5).
Uptake of U from contaminated soil
The addition of citric acid to the U-contaminated soil
increased U solubility to >110 mg U kg−1 soil, nearly
33% of the total U (Fig. 10). The addition of citric acid
also decreased the soil pH from 6.8 to the desired value
of 5.0. HEDTA had no effect on U solubility or the soil
pH. Neither soil amendment had a significant effect on
shoot biomass ( Fig. 11).
In terms of the plant accumulation of U, HEDTA did
not increase U uptake by beet (Fig. 12). However, the
addition of citric acid dramatically stimulated shoot U
accumulation by beet, by increasing U concentration
14-fold (from 15 to 209 mg U kg−1 DW ) The bioaccumulation ratio (shoot [ U ]/soil [ U ]) for beet (0.67) was at
least one order of magnitude greater than many of the
values previously reported in the literature for U (Ibrahim
and Whicker, 1988; Lakshmanan and Venkateswarlu,
1988; Sheppard and Evenden, 1988, 1992; Sheppard
et al., 1989).
Fig. 10. Soluble U in pots of U-contaminated soil amended with either
HEDTA or citric acid.
Fig. 12. Shoot U concentration for beet grown for 5 weeks in
U-contaminated soil amended 1 week prior to harvest with either
HEDTA or citric acid. Error bars denote the standard error of the
mean (n=5).
Uranium speciation and uptake
Discussion
The results presented here clearly indicate that the plantavailable form of U is the uranyl cation. Since this U
species is present in solution only at pH 5.5 or less,
U-contaminated soil with neutral to alkaline pH values
may require acidification in order for the phytoextraction of U to be successful. Geochemical studies of
U-contaminated sites such as the Fernald Environmental
Management Project suggested that U is present in the
soil chiefly as the anionic carbonate species (Lee and
Marsh, 1992). For several years, alkaline, carbonaceous
materials were added to this soil for erosion control and
road construction activities. Geochemical modelling has
shown, however, that the carbonate complexes that
subsequently formed are not only stable but potentially mobile, posing a threat to groundwater. Thus, the
addition of these carbonaceous materials was counterproductive. Similar management practices at other
U-contaminated sites could make phytoremediation more
difficult since the carbonate species do not appear to be
taken up and translocated to an appreciable extent. The
results presented here also indicate that citric acid can
greatly facilitate U bioavailability, even in a soil with a
neutral pH. This effect is due primarily to the solubilization and complexation of the uranyl cation by this organic
ligand and, to a lesser extent, by the change in pH ( Ebbs
et al., unpublished results).
One point that has not been established previously is
whether subsequent uptake of U into the roots of plants
involves transport of the U-citrate complex or just the
uranyl cation. The results of the hydroponic experiments
reported here suggest that it may be the latter species.
Furthermore, Munier-Lamy and Berthelin (1987) suggested that during the dissolution of U from soil, transient
complexes with simple molecules like organic acids may
form, preceding the formation of more stable polyanionic
or polycationic complexes. In a U-contaminated soil
following the addition of citric acid, there would be
extensive formation of these transient organic complexes,
more so than would be observed under normal soil
conditions. As these complexes break down in the soil,
most likely due to microbial degradation of the organic
acid, the dissociation of the U-citrate complex could
release large amounts of the uranyl cation. If this were
to occur in the rhizosphere, plant uptake of U could
occur before the uranyl cation has an opportunity to sorb
to soil particles and organic matter, or form additional
complexes in the soil solution.
Further research will be required, however, in order to
develop this information into an effective remediation
strategy for U-contaminated soils. The mechanism by
which citric acid increases U solubility in the soil and
uptake by plants needs to be studied in order to refine
addition of this compound to soil and to maximize U
1189
bioavailability. The effect of citric acid on the solubilization of U from additional U-contaminated soils, varying
in soil type, organic matter content, pH, and level of U
contamination, also needs to be examined to understand
more fully the role of other soil factors in U solubility as
they relate to phytoremediation.
Another aspect that requires investigation with respect
to the addition of citric acid to U-contaminated soil is
the long-term impact of this amendment on U speciation
in soil. The U-citrate complex, by its nature, is transient,
due to microbial and photodegradation. Francis et al.
(1992) and Dodge and Francis (1994) have shown, however, that degradation of U-citrate complexes can lead to
the formation of insoluble, non-leachable compounds
such as uranium trioxide. While this may facilitate U
recovery from aqueous systems, the conversion of soil U
to this form could hamper subsequent attempts to phytoextract U. The U chelated by citric acid following the
initial addition of this compound to U-contaminated soil
may originate in a pool of bound but potentially
exchangeable U. Dissociation of the U-citrate complex
following microbial or photodegradation could shift this
U into a pool that is less available than the pool from
which the U originated (i.e. to uranium trioxide). Thus,
while the first few croppings following citric acid addition
may effectively remove U from the soil, the long-term
effect of this addition may be to convert the U into a
form that does not respond to citric acid addition and
cannot be phytoremediated. Thus, the use of citric acid
may be effective in the short term, but may also create a
longer term problem by fixing U in the soil as uranium
trioxide. Given the problems that have developed from
the addition of alkaline, carbonaceous materials to
U-contaminated soil, further attempts at amend these
sites should be well planned so as not to compound the
existing problem.
Acknowledgements
The authors would like to thank Jay Cornish of MSE
Technology Applications Inc., Butte, MT, for contacting RMI
Environmental Services (Ashtabula, OH ) and securing the
U-contaminated soil used in this study. We would also like to
thank Dr Wendell Norvell of the US Plant, Soil, and Nutrition
Laboratory, USDA-ARS, for advice concerning the speciation
modelling and the pot study. This research was supported by a
grant from the United States Department of Energy- Division
of Energy Biosciences (Interagency Agreement DE-A102–
95ER21097).
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